1. Field of the Invention
The present invention relates to a method for inhibiting the polymerization of acrylic or methacrylic acid (acrylic acid and methacrylic acid being hereinafter collectively referred to as "(meth)acrylic acid") or esters thereof. More particularly, the present invention relates to a method for effectively inhibiting the polymerization of (meth)acrylic acid or esters thereof during their storage or upon their preparation, e.g., at a distillation stage.
2. Background Art
(Meth)acrylic acid or esters thereof have widely been utilized as raw materials for organic polymeric materials and a variety of organic materials. With the recent expansion of the field in which (meth)acrylic acid or esters thereof are utilized, those ones which have higher purities have been demanded. (Meth)acrylic acid and esters thereof tend to be naturally polymerized by light or heat. Therefore, there has been proposed to use various polymerization inhibitors in order to inhibit such a natural polymerization of (meth)acrylic acid or esters thereof during their storage or preparation. For instance, British Patent No. 1,127,127 discloses that an N-oxy radical such as tert-butylnitroxide or 2,2,6,6-tetramethyl-4-hydroxy-piperidine-1-oxyl, when used as a stabilizer for acrylic acid, exhibits a better polymerization-inhibiting effect than conventionally-known hydroquinone, phenothiazine and copper (II) chloride.
Further, U.S. Pat. No. 4,127,603 discloses a method for inhibiting the polymerization of methacrylic acid, in which 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl or 2,2,6,6-tetramethylpiperidine-1-oxyl is allowed to exist in the catalytic oxidation reaction of methacrolein to produce methacrylic acid; and Japanese Patent Publication No. 46, 496/1983 discloses a method for inhibiting the polymerization of (meth)acrylic acid esters, using an N-oxyl compound such as 2,2,5,5-tetramethyl-3-oxopyrrolidine-1-oxyl or 2,2,6,6-tetramethyl-4-acetoxypiperidine-1-oxyl.
Furthermore, European Patent Publication No. 620,206 discloses the use of 2,2,6,6-tetramethylpiperidine-1-oxyl, hydroquinone and phenothiazine in combination for inhibiting the polymerization of (meth)acrylic acid or esters thereof; and U.S. Pat. No. 5,504,243 teaches the polymerization-inhibiting effect which can be attained when an N-oxyl compound such as 2,2,6,6-tetramethyl-4-hydroxy-piperidine-1-oxyl is used in combination with other compounds such as manganese salt compounds, copper salt compounds, 2,2,6,6-tetramethylpiperidine compounds or nitroso compounds. In addition, British Patent Publication No. 2,285,983 discloses the use of phenothiazine, hydroquinone or the like in combination with copper dithiocarbamate, and metallic cobalt, cobalt oxide, cobalt acetate or the like, as a polymerization inhibitor for acrylic acid during its distillation process.
However, the above-described conventional methods for inhibiting the polymerization of (meth)acrylic acid or esters thereof have the problem that the polymerization is not always fully inhibited depending upon the conditions under which these methods are applied, especially under the high-temperature conditions upon the distillation of (meth)acrylic acid or esters thereof.